1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to an optical multiplexer/demultiplexer and an optical modulator which include coupled-waveguide grating devices and add/drop filters that, respectively, provide coarse and fine optical filtering.
2. Related Art
Silicon photonics is a promising technology that can be used to provide high-performance, chip-scale communication networks with low cost. Unlike on-chip electrical interconnects in which multiple metal layers are used to transport electrical signals, silicon-photonic interconnects typically use valuable silicon real estate to implement silicon waveguides that route optical signals. In particular, while some of the routing can be performed off-chip, in general these communication networks include significant on-chip routing. In order to minimize the impact on the silicon area, high-density integration is desirable for the silicon-photonic interconnects.
Dense wavelength division multiplexing (DWDM) is a technology for implementing on-chip optical communication networks because it offers the ability to effectively reduce the number of waveguides (and consequently to improve the integration density). In a DWDM link, signals are modulated on to optical carriers to produce optical signals that are conveyed using different wavelength channels in an optical interconnect. However, it has been difficult to implement DWDM links on silicon because of the challenges involved in implementing certain optical components, such as an optical multiplexer/demultiplexer.
A variety of techniques have been investigated for optical multiplexing/demultiplexing on silicon, including: an array-waveguide-grating (AWG) device, an Echelle-grating device, a Mach-Zehnder-based interleaver, cascaded ring-resonator add/drop filters, and coupled-waveguide grating devices. However, an optical multiplexer/demultiplexer based on AWG device, an Echelle-grating device or a Mach-Zehnder-based interleaver typically is large, which is not desirable for area-sensitive intra-chip applications. In principle, a ring-resonator add/drop filter or a coupled-waveguide grating device using a silicon waveguide that has a high index-of-refraction contrast has the potential to make a very compact optical multiplexer/demultiplexer if problems associated with these techniques can be addressed.
For example, an optical multiplexer/demultiplexer can be implemented by cascading multiple ring-resonator add/drop filters along a common bus optical waveguide, where a given ring resonator is tuned to align one of its resonant frequencies with a carrier wavelength associated with a particular optical channel. Note that, because of the periodic resonance structure of the ring resonators, these ring resonators usually have a free spectral range (FSR) that is larger than n·δλ, where n is the number of optical channels in a DWDM link and δλ is the wavelength spacing between the optical channels. Furthermore, because the FSR of a ring resonator is typically inversely proportional to its size, as the number of optical channels in a DWDM link is increased, the radius of the ring resonators used to implement the add/drop filters in optical multiplexers/demultiplexers usually needs to be decreased.
However, decreasing the radius of a ring resonator typically increases the bending loss, with a commensurate impact on the insertion loss and the Q of the add/drop filter. In addition, although an ultra-compact single ring-resonator add/drop filter having a radius of 3 μm has been demonstrated, thermally tuning such a ring resonator to align the add/drop filter with the carrier wavelength associated with a given optical channel may involve a temperature increase of hundreds of degrees. Such large temperature cycling can adversely impact the reliability of the ring resonator. As a consequence, the FSR of a ring-resonator-based add/drop filter is currently constrained, which can limit the ability to scale the number of optical channels in a DWDM link.
Similarly, coupled-waveguide grating devices can be used to implement compact optical multiplexers/demultiplexers. In this case, corrugation in these devices implements wavelength-selective coupling between the coupled-waveguide grating devices (which provide add/drop filtering) and a bus waveguide that conveys the optical signals associated with the optical channels in a DWDM link. However, such a wavelength-selective coupler typically works well for large bandwidths and has high losses for small bandwidths. As a consequence, a wavelength-selective coupler is often impractical as an add/drop filter in a DWDM link that includes many closely spaced optical channels. Thus, existing coupled-waveguide grating devices also limit the ability to scale the number of optical channels in a DWDM link.
Hence, what is needed is an optical multiplexer/demultiplexer without the above-described problems.